Electrochemical cells that incorporate ion conducting solid electrolytes have shown great promise for gaseous chemical synthesis applications. Electrochemical synthesis using such ion conducting solid electrolytes can produce high purity gases at higher reaction rates, with lower cost and without several of the chemical by-products and detrimental environmental impacts of traditional catalytic chemical synthesis processes.
For example, the traditional catalytic production of hydrogen gas (H2) and of ammonia (NH3), and the steps involved in their commercial scale implementation, are very energy intensive processes and produce massive amounts of carbon dioxide (CO2), a greenhouse gas widely acknowledged by the global scientific community as contributing to the warming of the earth's atmosphere and oceans.
Hydrogen gas (H2) is an important starting material for many industrial chemicals and also important as a primary fuel source in renewable energy production. Currently, most industrial hydrogen gas production involves catalytic steam reforming of a carbonaceous feed, such as natural gas, coal, liquefied petroleum gas, or the like, as follows:
                              C          n                ⁢                  H          m                    +                                    n            ⁢            H                    2                ⁢        O              ↔                            (                      n            +                          m              2                                )                ⁢                  H          2                    +              n        ⁢        CO                        CO      +                        H          2                ⁢        O              ↔                  H        2            +              CO        2            This process typically requires high temperatures such as 700-1000° C.) and, in practice, involves various additional steps, such as removing sulfur from the carbonaceous feed.
Ammonia (NH3) is one of the most highly produced inorganic chemicals in the world because of its many commercial uses, such as in fertilizers, explosives and polymers. Modern commercial production of ammonia typically utilizes some variation of the Haber-Bosch process. The Haber-Bosch process involves the reaction of gaseous nitrogen (N2) and hydrogen (H2) on an iron-based catalyst at high pressures (such as 150-300 bar) and high temperature (such as 400-500° C.), as follows:N2+3H2→2NH3 In modern ammonia-producing plants, the nitrogen feed typically derives from atmospheric air but the hydrogen feed typically derives from catalytic steam reforming of a carbonaceous feed stock, discussed above. In practice, implementation of this process requires various other steps, such as separating and purifying the hydrogen before it can be used.
Commercial scale production of industrially important chemicals, such as hydrogen and ammonia, may be achieved more efficiently and cost effectively by electrochemical synthesis than by the traditional catalytic processes such as those discussed above. For example, hydrogen gas can be produced by direct electrolysis of steam (H20) without requiring a carbonaceous feed source and the commensurate production of carbon dioxide. Ammonia may be electrochemically produced by directly reacting the hydrogen with nitrogen, eliminating the need for intermediate steps to separate and purify the hydrogen before use in the synthesis reaction.
Electrochemical synthesis is typically carried out using an electrochemical cell that incorporate two electrodes (an anode and a cathode) and an electrolyte that separates the two electrodes. As used in electrochemical synthesis applications, the two electrodes are connected via electronic circuitry to a power source, and the electrolyte typically is a material that conducts ionic species but not electrons nor non-ionized species, such as the initial chemical reactants and final chemical products. When a voltage is applied across the two electrodes, a reactant is dissociated and ionized at one electrode, and the ionized reactant species migrates through the electrolyte toward the opposite electrode, where it reacts (in some cases with a second reactant that is present at the opposite electrode) to form the desired reaction product. The materials and configuration of the electrodes and electrolyte are selected and optimized depending on the desired electrochemical synthesis reaction.
For electrochemical synthesis to be widely applicable and commercially viable, there exists a need for electrochemical cells that may be fabricated using a variety of materials and in various configurations, depending on the desired electrochemical synthesis reaction, in a cost-effective and scalable way. Moreover, it is desirable for the electrochemical cells to have the structural and chemical stability and durability to withstand the potentially severe temperatures, pressures and chemical environments in which they would operate.